By Angela Nelson
In March 2024, for the first time in the United States, the highly pathogenic avian influenza (HPAI) H5N1 virus was recognized as having crossed over from birds into cattle. Experts believe the virus was transmitted a month or two beforehand from birds to dairy cows in the Texas panhandle, and then spread as the cows were moved to dairy farms in other parts of the country.
To date, dairy cows in 17 states have tested positive for HPAI, with more than 970 herds affected, according to the Centers for Disease Control and Prevention (CDC). In addition to the confirmed crossover event in Texas, two additional crossover events were confirmed in Colorado and Arizona.
There have been 70 confirmed human cases of HPAI infection, at least 40 of which were in people with exposure to dairy cattle, the CDC said, and most of the other cases occurred in people with exposure to poultry farms. One person died from avian influenza after exposure to "a combination of a non-commercial backyard flock and wild birds," according to the Louisiana Department of Health.
However, there have been no human fatalities due to HPAI contracted from dairy cattle. HPAI has been found in more than 200 mammals, such as seals, bears, and cats, since 2022, according to the CDC.
In December 2024, another human case was suspected in a child in California who had consumed raw milk and tested positive. However, a second test on the child was negative. When milk is pasteurized, the heating process kills the virus and any other harmful bacteria, but raw milk is not pasteurized.
David Hernke, D.V.M., V08, is an assistant clinical professor in the Department of Ambulatory Medicine and Theriogenology at Cummings School of Veterinary Medicine at Tufts University, who's based in Woodstock, Connecticut with the Tufts Veterinary Field Service. The VFS is an ambulatory large animal veterinary practice that works with farms throughout New England to keep cattle, sheep, and other animals healthy.
Hernke is a member of the American Association of Bovine Practitioners (AABP), and he focuses on dairy cow health and dairy production medicine in New England, where dairy cows have not yet been affected by HPAI.
"It is viscerally concerning to know that avian influenza, especially highly pathogenic avian influenza, has gotten into a novel species and is moving around," he said.
Hernke recently spoke with Tufts Now about avian influenza in cows, including the biosecurity measures in place to protect the cattle at Cummings School, and how he advises his clients to keep their livestock safe.
What do we know about how HPAI spreads to and among cows?
The AABP offers monthly updates about the outbreak, with an on-the-ground look at what's happening which is really helpful for those of us who are not directly involved. They have said that, so far, three crossover events are responsible for all cases of dairy cows with HPAI.
In other words, birds transmitted the virus to cattle in Texas, Colorado, and Arizona, which spread the virus as they were moved throughout the U.S., and the cattle then spread it among the herd. Birds did not infect each herd or cow separately, though more crossover events could happen.
The unique thing about this avian influenza is that it has a tropism for the udder, so it is shed in milk fairly heavily. When we are doing testing on our clients' farms, milk samples are what we need to submit to the respective state diagnostic lab because milk samples have the best sensitivity for picking up detections.
We can test nasal swabs, but the virus does not seem to shed as heavily in respiratory secretions. Which means that the focus for testing, to this point, has been on lactating dairy cows, not other types of cattle, such as non-lactating female or male cattle or feeder beef. So, anywhere there's milk or milk droplets, especially in milk parlors or on equipment at the dairy farms, there is risk of transmission to other animals or even humans.
Interestingly, in the initial outbreak, one of the practitioners in the area noted a number of dead cats at the dairies where cattle tested positive for avian influenza. They submitted some of the deceased cats to a national animal health lab in Iowa, which found large amounts of this virus in the cats, likely because the cats drank the infected cows' milk.
What questions do your clients have about keeping their dairy cows safe?
Our clients have lots of questions, especially about biosecurity, and that's really where the rubber hits the road for Tufts' field service. We work with dairies throughout Connecticut, Rhode Island, and central Massachusetts.
The biggest concern is the area where dairies keep cows that are sick. At the dairies, if a cow is recognized to not be well for any reason, whether it's respiratory issues or an infection of the udder, the cows are moved out of the general population and into a separate area for treatment, out of an abundance of caution for the other animals.
Cows manifesting influenza symptoms would end up in that area, and those populations of cows are milked in a separate parlor, so they're not commingled with healthy cows being milked for public consumption.
What other measures have your clients taken to protect their workers and cattle?
Many of our clients increased biosecurity in their milk parlors for the safety of the employees. Milkers have always worn gloves; that's part of the normal milking process, because people can transmit disease to cows. But they increased it with face shields, much like the personal protection worn during the height of the COVID-19 pandemic. Milkers also wear waterproof aprons to make disinfection easier and more effective.
Another step many of our clients took was to tighten access to their animal areas. Farm employees and visitors scan through entrances, so there's real-time tracking of who's coming and going in the animal areas and the feed areas, as well.
In the past, most of our clients never really allowed the public to just walk through the dairy farms anyway. However, a few of them conduct tours for the public, who come to visit the farm but do not go inside the animal areas. The farmers felt that these tours are part of their overall public education mission, and they asked us about the risks of those tours, which we believe are low.
Are dairy farms required by state or federal governments to test their milk or herds for HPAI?
We've been fortunate that we haven't had the virus in cattle in the Northeast U.S. yet. As part of surveillance efforts, Massachusetts recently concluded milk testing on all the dairies, for which the state conducted three tests per dairy. They all came back negative. Connecticut is discussing similar testing efforts.
Right now, a lot of the testing programs are voluntary for HPAI and cattle—unless a dairy tests positive, at which point testing becomes mandatory. However, the U.S. Department of Agriculture (USDA) requires that any lactating cows crossing state lines must test negative for HPAI. They've started a program where if a dairy participates in a surveillance program of their bulk tank, with several negative tests over a period of time, then the dairy no longer has to test individual cows that are moving. The bulk tank is where all the milk goes from the lactating cows on each farm, so it's a good way to test a lot of individuals at once. The program is meant to encourage dairies to test their tanks.
The USDA's Animal and Plant Health Inspection Service (APHIS) recently issued a federal order requiring that raw, unpasteurized milk samples nationwide be collected and shared with the USDA for testing, as part of its National Milk Testing Strategy.
What happens to the sick cows?
I recently attended a webinar by the AABP during which they showed what the results and ramifications have been. Initially, the mortality is remarkably low due to the disease. In fact, it was reported in the initial outbreak that almost no cows died directly as a result of having been infected with avian influenza.
However, because dairy cows are production animals, they have to make milk, and they have to get pregnant in order to stay at the herd. Analysis has shown a marked change in the productivity of the cattle that have been infected. Those animals are more likely to not continue to be dairy cows, because their production never comes back to what it was. They estimate the cost to the dairy per case of the virus was about $900 because of increased culling, or removal from the herd, and loss of milk.
There's also a reproductive effect. I just attended a seminar that provided an economic analysis of a herd that was infected in Ohio. If you looked at the data alone, you might have thought, "Wow, the mortality was really low. It didn't affect the farm that much." But drilling down, they found a much higher rate of culling, for production reasons, of the cows that were diagnosed positive for avian influenza. So, at least currently, they don't die from the disease, but it does negatively affect them in the long term. As more herds are affected, we may find that this changes.
Recently there's been talka about potential for vaccination, because that's a cornerstone of prevention of influenza in other species. We're not there yet, but the precedent is there. We vaccinate cows for a lot of other diseases.
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